Electronic packages, as mentioned herein, are known which include a circuitized substrate such as a printed circuit board, chip carrier, or other laminated structure as the base for having one or more semiconductor chips mounted thereon and electrically coupled to other components on the substrate as well as within the overall electronic assembly (e.g., a personal computer, mainframe or server) in which the package is positioned. Many of today's packages include, as also indicated, a substrate of laminate construction in which various conductive and dielectric layers are stacked together with selected elements of the conductive layers electrically connected to corresponding elements of other layers, typically through a conductive “via.” The dielectric layers are typically of a fiberglass-reinforced epoxy resin (also known as “FR4”) or like material while the conductive layers are typically copper, and may be in the form of signal, power or ground layers. Laminate structures have been and continue to be developed for many applications. These are displacing ceramic substrates in many chip carrier applications, because of reduced cost and enhanced electrical performance. The use of a multi-layered interconnect structure such as an organic, laminate chip carrier for interconnecting semiconductor chips to one another and to a printed circuit board in an electronic package introduces many challenges, one of which is the reliability of the connection joints between the semiconductor chip and the organic chip carrier, and another of which is the reliability of the connection joints between the organic chip carrier and the printed circuit board. Other challenges are identified below.
As semiconductor chip input/output (I/O) counts increase beyond the capability of peripheral lead devices and as the need for both semiconductor chip and printed circuit board miniaturization increases, area array interconnects are the preferred method for making large numbers of connections between one or more semiconductor chips on an organic chip carrier or printed circuit board, as well as between modules (packages) mounted on a printed circuit board (such modules are also referred to as packages and may include a substrate with one or more chips and a covering housing and/or heat sink, in addition to other elements). If the coefficient of thermal expansion (CTE) of the semiconductor chip, the organic chip carrier, and the printed circuit board are substantially different from one another, industry standard semiconductor chip array interconnections to the organic chip carrier can exhibit high stress during operation (thermal cycling). Similarly, the industry standard ball grid array (BGA) interconnections between the organic chip carrier and printed circuit board can also exhibit high stress during operation. Significant reliability concerns may then become manifest by failure of the connections or even failure of the integrity of the semiconductor chip (chip cracking). These reliability concerns significantly inhibit design flexibility. For example, semiconductor chip sizes may be limited or interconnect sizes, shapes and spacing may have to be customized beyond industry standards to reduce these stresses. These limitations may limit the electrical performance advantages of the organic electronic package or add significant cost to the electronic package. Typically a semiconductor chip has a CTE of 2-3 parts per million per degree Celsius (ppm/° C.) while a standard printed circuit board has a much greater CTE of 17-20 ppm/° C. A chip carrier, with usually a plurality of dielectric layers and conductive layers as part thereof, typically has a CTE somewhere between these two structures.
One example of an organic chip carrier designed to overcome such CTE and related problems is defined in U.S. Pat. No. 6,351,393 (J. S. Kresge et al) which includes a specific thermal internally conductive layer designed to prevent failure between the single chip and the carrier solder connections, and those between the carrier and base substrate (e.g., PCB) on which it is positioned.
Another significant concern with respect to high density wiring arrays is line skew, in which lines of different lengths between respective chip and substrate contact sites, as well as between different chip sites, cause the signals there-through to arrive at the designated locations at different times. This in turn may degrade system performance as well as lead to errors in data communication, two problems especially relevant to chip packages with a plurality of chips such as a main processor chip and adjacent memory chips. The extreme closeness of positioning between such chips necessitates that the several connecting lines between the chips be positioned in a minimum of spacing both on and within the laminate substrate. Signal speed requirements for such packages are also extremely high. To satisfy such demands in such limited spacing requirements, the circuit designer is in turn limited as to how best to place all such lines, and still assure routing between all required points. To accomplish this, he/she has found it necessary to often connect the shortest distances first with short lines and then attempt to accomplish all additional connections with whatever line scheme will satisfy the grid. As explained below, such long lines extend considerably outwardly, especially when coupling component sites having several individual contacts as part thereof. The result, as explained above, is the undesirable signal arrival times and corresponding problems associated therewith.
The following patents describe line skew for circuits and various means for dealing with same.
In U.S. Pat. No. 6,836,163, issued Dec. 28, 2004, there is described a differential output structure with minimal skew and less process variations. According to one embodiment, the structure includes an input line, an output driver and a sync circuit. The input line includes first and second paths. The first path has an input end for receiving input signals. The first path also has an output end and includes at least one driving element. The second path has an input end coupled to the input end of the first path for receiving the input signals. The second path also has an output end. The output driver is coupled to the output ends of the first and second paths and is configured to provide differential outputs. The sync circuit is coupled between the first and second paths and is configured to synchronize the speed of signals traveling on the two paths.
In U.S. Pat. No. 6,683,503, issued Jan. 27, 2004, there is described an oscillation circuit which provides clock signals and a clock distribution of circuits having low skew and low jitter to logic circuits and memory circuits of a microprocessor or the like. The oscillation circuit is in a semiconductor integrated device having a plurality of oscillators each having an oscillation node, where the oscillation nodes of each of the oscillators are connected together by a conductive wiring line that may be a closed loop. The oscillators are synchronized to oscillate at substantially the same frequency. The oscillators are connected to the conductive wiring line at connecting points having substantially the same interval of conductive wiring lengths between the connection points, which leads to synchronizing the oscillators to oscillate with a substantially identical phase. The conductive line can also be formed in the shape of a mesh. The oscillators are ring oscillation circuits having inverters connected in a ring shape where an output of at least one inverter of each ring oscillation circuit is connected to the conductive wiring. Alternatively, the oscillators may be delay lines having multistage connected inverters with at least one inverter connected to the conductive wiring line.
In U.S. Pat. No. 6,442,057, issued Aug. 27, 2002, there is described a memory “module” for preventing bus line skew. The memory module includes a PCB, memory chips, module tabs and bus lines. The memory chips are disposed on the PCB, and the module tabs are disposed at one edge of the PCB. The bus lines are connected to the module tabs, respectively, and are connected to the memory chips. Each of the bus lines is a closed loop and is connected to the memory chips through a circuitous or roundabout path which includes first and second paths of, in general, different lengths. The first and second paths of the roundabout path branch from each other at a position on the closed loop.
In U.S. Pat. No. 6,161,215, issued Dec. 12, 2000, signal delay and skew within an integrated circuit are minimized when signals are distributed to distant points of an integrated circuit through a layer of its package, and traces in the package layer are etched and treated as transmission lines. As mentioned, a signal is driven through a first connection between an integrated circuit and an integrated circuit package layer. The signal is then distributed to one or more additional connections between the integrated circuit package layers, by means of point-to-point transmission lines formed in the package layer, each of the transmission lines being terminated at one or both ends by impedances which are substantially matched to the characteristic impedance of the transmission line to which these are attached. The signal is then received into the integrated circuit through the one or more additional connections between the integrated circuit and the package layer.
In U.S. Pat. No. 5,849,610, issued Dec. 15, 1998, there is described a methodology of constructing a planar equal path length clock tree. Prior computer-generated methods of creating low-skew clock trees required that clock sinks be uniformly distributed throughout the circuit. Moreover, the tree produced would often be non-planar, thus increasing layout design complexity and cost. This patent describes a method of automatically producing a planar clock tree with equal path lengths from each clock sink to the clock source. A first branch wire is formed between the clock source and the clock sink that is a farthest distance from the clock source. Thereafter, the remaining uncoupled clock sinks are coupled to the clock tree according to a maximum rule and a minimum rule. Thus a planar equal path length clock tree is formed. The planar equal path length clock tree is transformed in to a rectilinear clock tree, including horizontal and vertical wires, by using a line search algorithm. The rectilinear clock tree may then be optimized for tolerable clock skew by a cut-and-link method.
Examples of other packaging structures are shown and described in the following documents:
U.S. Patents4,882,454November 1989Peterson et al5,072,075December 1991Lee et al5,121,190June 1992Hsiao et al5,483,421January 1996Gedney et al5,615,087March 1997Wieloch5,661,089August 1997Wilson5,798,563August 1998Fielchenfeld et al5,838,063November 1998Sylvester5,894,173April 1999Jacobs et al5,900,675May 1999Appelt et al5,926,377July 1999Nakao et al5,982,630November 1999Bhatia
Foreign Patent DocumentsJP1-307294December 1989JP6-112271April 1994JP9-232376September 1997JP10-209347August 1998JP11-087560March 1999JP2000-022071January 2000JP2000-024150January 2000
As mentioned in the last two of the three foregoing pending patent applications, to increase the operational characteristics of electronic packages, the addition of more than one chip to the upper surface of a chip substrate is known. (Both of these pending applications also define examples of how such chips may be positioned and coupled, using non-ceramic materials for the dielectric layering.) Because use of added chips in a close spacing relationship results in increased operating temperatures from the chips, many structures (other than those in the pending applications) require the much higher temperature compensating substrate material ceramic as the underlying substrate material, with various examples described in the following IBM Technical Disclosure Bulletins (TDBs):
July 1978Multi Chip Cooling Platepp 745-746February 1982Simultaneous Chip Placement -pp 4647-4649Multi-Chip ModulesNovember 1987High Performance Multi-Chippp 437-439ModuleAugust 1988Low-Cost, High-Power, Multi-Chippp 451-452Module DesignSeptember 1993Thermally Conductive Substratepp 623-624Mounted Multi-Chip Module Cap
As mentioned, the use of ceramic poses many problems, a primary one of which is handling. Ceramic is a relatively brittle material capable of cracking and chipping if handled improperly during manufacture and shipping. Ceramic is also a relatively difficult material to process, especially to the multi-depth level where several individual layers of insulative and interconnecting conductive materials are needed to satisfy many operational requirements. To overcome these problems, the pending applications define structures which do not require ceramic insulative material, but which instead are capable of using non-ceramics.
In addition to the above-cited two pending applications, chip carriers of non-ceramic material have been proposed, but these typically possess various drawbacks. In U.S. Pat. No. 5,574,630, for example, three chips are mounted on a substrate comprised of silica-filled polytetrafluoroethylene (PTFE) but require individual vias to pass through the carrier's entire this structure in turn mandates utilization of a complex “power/ground assembly” of several layers having specific CTEs and other properties, thus resulting in a very expensive final assembly and one that is relatively difficult to construct.
Yet another non-ceramic substrate embodiment for having more than one chip thereon is described in U.S. Pat. No. 6,246,010. Unfortunately, the substrates require semiconductor chips which are extremely thin (less than 100 μm, preferably less than 50 μm, and “most preferably” less than 20 μm). Understandably, such thinned chips are incapable of adequately providing the much greater operational capabilities as required by today's more powerful chips (e.g., those of the application specific integrated circuit (ASIC) variety). Typically, such chips operate at much higher temperatures than other types (e.g., those of the dynamic random access memory (DRAM) variety).
A typical high performance PCB, prior to the one defined in U.S. Pat. No. 6,828,514 (cited above), has not been able to provide wiring densities beyond a certain point due to limitations imposed by the direct current (DC) resistance maximum in connections between components (especially chips). Similarly, high speed signals demand wider lines than normal PCB lines to minimize the “skin effect” losses in long lines. To produce a PCB with all wide lines would be impractical, primarily because of the resulting excessive thickness needed for the final board. Such increased thicknesses are obviously unacceptable from a design standpoint. As mentioned, high speed signal requirement are especially typical in chip carrier applications, especially those which use at least one main processor chip and one or more added memory chips which operate in combination therewith.
The unique characteristics of the PCB in U.S. Pat. No. 6,828,514 allow it to be able to assure high frequency connections while still utilizing relatively standard PCB manufacturing processes to produce the final structure. In this pending application, a portion of the PCB is dedicated to utilizing relatively wider lines than the remaining, lower portion of the PCB, which includes lines and spacings known in the PCB field.
All of the above documents are incorporated herein by reference.
It is believed that a method of making an electronic package which can utilize a substrate with high density wiring and which can effectively interconnect two or more chips mounted on the substrate such that high speed signals can pass through the closely spaced lines and assure substantially similar arrival times in a manner not suggested in the cited documents above would constitute a significant advancement in the art. It is further believed that such a package, which can also provide for effective coupling between two or more conductive signal layers within the package's substrate, would also constitute an art advancement. Equally important, it is believed that such a package which can overcome many of the disadvantages cited above for many known multi-chip structures, including particularly those associated with CTE mismatch, would truly be an art advancement.